Retroreflective appliques have the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has led to widespread use of retroreflective appliques on clothing. Persons who work or exercise near motor vehicle traffic need to be conspicuously visible so that they do not get struck by passing motor vehicles. Retroreflective appliques serve the purpose of highlighting a person's presence by retroreflecting light from motor vehicle headlamps.
A retroreflective applique typically comprises a layer of optical elements, a polymeric binder layer, and a specular reflective layer. The optical elements usually are transparent microspheres that are partially embedded in the polymeric binder layer such that a substantial portion of each microsphere protrudes from the polymeric binder layer. The specular reflective layer typically comprises aluminum, silver, or a dielectric mirror, and this reflective layer is disposed on the portion of the transparent microsphere which is embedded in the polymeric binder layer. Light striking the front surface of the retroreflective applique passes through the transparent microspheres, is reflected by the specular reflective layer, and is collimated by the transparent microspheres to travel back in the direction of the light source.
Retroreflective appliques must be able to withstand laundering conditions when displayed on clothing. If the retroreflective applique is not durable to laundering and its retroreflectivity becomes substantially diminished after repeated washings, the clothing cannot continue to serve its safety function by highlighting the wearer's presence. Investigators in the retroreflective applique art have pursued an ongoing goal of developing launderably-durable retroreflective appliques so that persons wearing retroreflective clothing can continue to be conspicuously visible after the clothing has been worn and cleaned many times.
In U.S. Pat. No. 4,763,985 to Bingham, a launderable retroreflective applique is disclosed that comprises a layer of transparent microspheres, a specular reflective layer optically connected to each microsphere, and a binder layer into which the microspheres are partially embedded. Resins disclosed as being suitable for use as binder layers include polyurethane, polyesters, polyvinyl acetate, polyvinyl chloride, acrylics, or combinations thereof. The specular reflective layers are composed of two succeeding layers of dielectric material. The layers have varying refractive indices and are composed of a variety of binary metal compounds including oxides, sulfides, and fluorides. Although the retroreflective applique of U.S. Pat. No. 4,763,985 is disclosed to be durable under laundering conditions, the durability is only provided for appliques that employ dielectric reflectors. The patent does not disclose how to provide a launderable retroreflective sheeting that employs an elemental aluminum or an elemental silver metal reflector.
In U.S. Pat. No. 5,200,262 to Li, a launderable retroreflective applique is disclosed that can employ a reflector that comprises elemental aluminum or elemental silver on the backside of the microspheres. The applique comprises a monolayer of metal-coated microspheres partially embedded in and partially protruding from a binder layer that comprises a flexible polymer having hydrogen functionalities and one or more isocyanate-functional silane coupling agents. The disclosed flexible polymers that possess hydrogen functionalities are crosslinked, flexible urethane-based polymers such as isocyanate-cured polymers or one or two component polyurethanes and polyols. This retroreflective applique can provide excellent laundering durability: it can withstand industrial laundering conditions, which involve wash temperatures as high as 40 to 90.degree. C. (105 to 190.degree. F.) and pH values of 10 to 12.5. Although the applique demonstrates superior laundering durability, it does not do so without a price. The materials employed in the binder layer are relatively expensive, and they typically are solution-coated onto the microspheres, which requires use of a solvent that adds to the processing costs and that must be reclaimed or disposed of in an environmentally sound manner.
In U.S. Pat. No. 5,283,101 to Li, a launderable retroreflective applique is disclosed which comprises a binder layer formed from an electron-beam curable polymer and typically one or more crosslinkers and silane coupling agents. Electron-beam curable polymers disclosed in Li include chlorosulfonated polyethylenes, ethylene copolymers comprising at least about 70 weight percent of polyethylene such as ethylene/vinyl acetate, ethylene/acrylate, and ethylene/acrylic acid, and poly(ethylene-co-propylene-co-diene) polymers. Glass microspheres are embedded in the cured binder layer, and a specular reflective metal layer is disposed on the embedded portions thereof. The retroreflective appliques have been shown to be durable under industrial laundering conditions; however, the applique's binder layer typically includes a crosslinker and a coupling agent and requires the step of curing the polymer with an electron beam. Additionally, a solvent is employed to coat the binder layer onto the microspheres, which needs to be reclaimed or disposed of in an environmentally sound manner.